In January 1969, U.S. Pat. Nos. 3,421,692; 3,421,699 and 3,425,058 issued to Robert S. Babington, the present applicant, and his co-inventors. These patents disclose a type of liquid atomization apparatus which is particularly useful in liquid fuel burners. The principle involved in the apparatus, now frequently referred to as the "Babington principle," is that of preparing a liquid for atomization by causing it to spread out in a free-flowing thin film over the exterior surface of a plenum having an exterior wall which defines an atomizer bulb and contains at least one aperture. When gas is introduced into the plenum, it escapes through the aperture and thereby creates a very uniform spray of small liquid particles. By varying the number of apertures, the configuration of the apertures, the shape and characteristic of the surface, the velocity and amount of liquid supplied to the surface, and by controlling the gas pressure within the plenum, the quantity and quality of the resultant spray can be adjusted as desired for a particular burner application. Various arrangements of such atomization apparatus have been disclosed in other U.S. patents issued to the present applicant, namely U.S. Pat. Nos. 3,751,210; 3,864,326; 4,155,700; and 4,298,338. The disclosures of the patents mentioned in this paragraph are specifically incorporated by reference into this application.
FIG. 1 of this application illustrates a liquid fuel atomizing apparatus of the general type disclosed in the previously mentioned patents, which operates in accordance with the Babington principle. An enclosed housing 10, typically cylindrical in configuration, defines an atomizing chamber 12 having a front or dividing wall 14 through which passes a conical discharge opening or discharge cone 16. Housing 10 also includes a back wall 18 from which is supported an atomizer bulb 20 comprising an enveloping exterior wall 22 which defines an internal plenum (not illustrated) and tapers toward a frontal aperture 24. In a typical prior art application in which atomizer bulb 20 comprises a spherical tip of approximately 12.7 mm (0.500 inch) diameter, aperture 24 was spaced approximately 6.35 mm (0.250 inch) from the front exit face of discharge cone 16. In such an example, the inlet diameter of cone 16 was approximately 20.83 mm (0.820 inch) and the outlet diameter was approximately 14.73 mm (0.580 inch).
A source 26 of high pressure air is connected to the plenum defined by exterior wall 22 by means of a conduit 28 so that in operation a flow of air is caused to pass through aperture 24. Positioned above atomizer bulb 20 is a liquid fuel feed tube 30 which in the past has had a circular cross-section but may also have other cross-sections without departing from the scope of the present invention. Liquid fuel drawn from a sump 32 through a conduit 34 by a pump 36 is caused to flow through a further conduit 38 into feed tube 30. From there, the fuel flows over atomizer bulb 20 and forms a film of liquid which completely covers the surface of bulb 20. Of the fuel flowing over the surface of the atomizer bulb, that portion which is not atomized flows from the lower side of bulb 20 as a stream 40 which is directed back to sump 32 through conduit 42, as illustrated. As air flows through aperture 24, the film of liquid continuously forming at the aperture is continuously broken into tiny droplets of liquid which move away in the form of a fine, essentially conical spray 44 of atomized fuel.
In such prior art systems, spray 44 includes some stray or satellite droplets which diverge from the conical flow path illustrated. As a result, the conical wall of discharge cone 16 tends to become wetted and a small amount of liquid fuel flows backward into atomizing chamber 12 and also returns to sump 32 via conduit 42. To complete the schematic illustration of such a prior art fuel burner, FIG. 1 also shows an ignition control 45 and an igniter 46, the latter being located at the outer periphery of spray 44 at a downstream location in order to ignite the fuel in a manner described more completely in the previously-mentioned patents. Ignition of the fuel thus occurs within a flame tube 48, a greatly shortened version of which is shown in FIG. 1.
In order to minimize the possibility that combustion might occur within atomizing chamber 12, a condition known as "burn-back," it is known to provide a flow of air at a pressure slightly greater than atmospheric into chamber 12, past atomizer bulb 20 and through discharge cone 16 along with spray 44. A pair of openings 50 may be used to provide this flow of air usually from a blower that operates at substantially less pressure than high pressure source 26 which supplies air to atomizer 20. In such a prior art apparatus, the flame front F, that is, the point at which a flame is first visible, sometimes has been observed at a point within discharge cone 16, as illustrated.
While this type of prior art liquid fuel burner has been shown to be a practical and efficient burner for domestic and industrial use, some problems, or what have been perceived as problems, have continued to exist. Concern has arisen on the part of some that under particularly adverse conditions, burn-back into atomizing chamber 12 might yet occur. For example, if the pressure and hence the flow of air through atomizer bulb 20 were to decrease in conjunction with insufficient ventilation of chamber 12, flame front F might actually move within atomizing chamber 12, a condition which might lead to burn-back during operation or just after shutdown. A sudden reduction or total cessation of the flow of air flow through conduits 50 as might be experienced if the blower inlet were accidentally closed, might also result in a burn-back situation. Since a portion of the fuel used in these burners is continuously recirculated, burn-back might cause the temperature of the fuel to increase to levels above the flash point. Also, pressure surges in flame tube 48, caused for example by downdrafts in the chimney of a domestic furnace, have been suggested as a possible cause of burn-back into atomizing chamber 12, especially if such a downdraft were to occur at the same time as the aforementioned irregularities.
The flow of pressurized air through conduits 50 into atomizing chamber 12 has been recognized for some time as a means for combatting these potential causes of burn-back. The flow through the atomizing chamber helps to reduce the temperature of the fuel, satisfies the entrainment needs of the high velocity jet of air issuing from aperture 24, promotes mixing of fuel and air and also tends to promote a more controllable location for flame front F. These entrainment needs arise because the jet of air and liquid produces a zone of reduced pressure just outside aperture 24. Without the flow of air through conduits 50, part of which is entrained at the zone of reduced pressure, air or combustion products would be drawn backward from flame tube 48 into atomizing chamber 12 to the zone of reduced pressure, thereby interfering with the formation of spray 44 and increasing the likelihood of burnback. However, as the velocity of air flowing over atomizer bulb 20 increases in such prior art burners, ripples and other flow irregularities can occur in the film flowing over the atomizer bulb, which can result in undesirable carry-over of raw fuel into flame tube 48, or irregular atomizing producing large droplets, or both. It is also thought that some fuel may be torn from stream 40 and carried into flame tube 48 when the airflow through the atomizing chamber is too high. In an effort to control the velocity of the air passing over the tip of atomizer bulb 20, the tip of the bulb has been spaced as much as 6.35 mm (0.250 inch) from the exit face of discharge cone 16, as indicated previously. However, care has been taken not to move the tip of the bulb so far from the discharge cone that flame front F moves into the atomizing chamber, or excessive fuel impinges on the walls of cone 16.
So that such prior art liquid fuel burners and liquid atomizers can have the widest possible range of applications, another continuing problem has been to provide the maximum possible variation in the volumetric flow rate of the atomized fuel or other liquid in spray 44 between the lowest and highest flow rates required. For example, flow rates as low as 0.3785 liter (0.1 gallon) per hour may be required for some applications and as high as 3.785 liters (1.0 gallon) per hour may be required for others.
Once the particular geometry for a given prior art atomizer apparatus has been selected, however, changes in the flow rate of the atomized liquid in spray 44 must be made primarily by adjusting the flow rate of liquid onto the atomizer bulb. For the lowest rates desired, the liquid film thickness at the aperture preferably would be the thinnest achievable while still maintaining a continuous film over the exterior surface of the atomizer bulb. On the other hand, to provide higher flow rates of the atomized liquid, it is necessary to increase the thickness of the film at the aperture without increasing it so much that undesirably large liquid droplets are formed.
In prior art atomizers of the type shown in FIG. 1, a single liquid feed tube has been positioned above each atomizer bulb at a distance of approximately 3.175 to 9.25 mm (0.125 to 0.375 inch) so that a variable flow rate of atomized fuel from about 0.757 to 2.271 liters (0.2 to about 0.6 gallon) per hour has been achievable. In such a case, approximately 0.56 to 0.7 cu.m./m (20 to 25 cfm) is needed for combustion air in flame tube 48. About 10% of this amount is aspirated by the jet pump action of the atomizer bulb. Due to the presence of a relatively high velocity of air over the surface of atomizer bulb 20, which could be as high as 9.14 to 10.67 m/sec (30 to 35 ft/sec), the achievement of significantly thinner or thicker films on the atomizing bulb has been difficult without undesirable ripples in the film. Various applications have remained, however, in which atomized liquid flow rates above and below the range previously mentioned have been desired but have not been reliably achievable.